(1) Field of the Invention
The present invention relates to an organic electroluminescence display panels and methods of driving the same, and particularly relates to an organic electroluminescence display panel that uses an active-matrix drive circuit and to a method of driving the same.
(2) Description of the Related Art
Display panels using organic electroluminescence (EL) elements are known as display panels that use current-driven luminescence elements. An organic EL display panel that uses such self-luminous organic EL elements does not require backlights that are needed in liquid crystal display panels, and is thus well-suited for increasing device thinness. Furthermore, since viewing angle is not restricted, practical application thereof as a next-generation display panel is expected. Furthermore, the organic EL elements used in the organic EL display panel are different from liquid crystal cells which are controlled according to the voltage applied thereto, in that the luminance of the respective luminescence elements is controlled according to the value of the current flowing thereto.
In organic EL display devices, organic EL elements included in pixels are normally arranged in a matrix. In an organic EL display referred to as a passive-matrix organic EL display, an organic EL element is provided at each crosspoint between row electrodes (scanning lines) and column electrodes (data lines), and such organic EL elements are driven by applying a voltage equivalent to a data signal, between a selected row electrode and the column electrodes.
On the other hand, in an organic EL display panel referred to as an active-matrix organic EL display device, a switching thin film transistor (TFT) is provided in each crosspoint between scanning lines and data lines, the gate of a drive element is connected to the switching TFT, the switching TFT is turned ON through a selected scanning line so as to input a data signal from a signal line to the drive TFT, and an organic EL element is driven by such drive TFT.
Unlike in the passive-matrix organic EL display panel where, only during the period in which each of the row electrodes (scanning lines) is selected, does the organic EL element connected to the selected row electrode produce luminescence, in the active-matrix organic EL display panel, it is possible to cause the organic EL element to produce luminescence until a subsequent scan (selection), and thus a reduction in display luminance is not incurred even when the number of scanning lines increases. With this point, the active-matrix driving method has an advantage in realizing a large-screen and high-definition display panel.
On the other hand, in an organic EL display panel using current-driven organic EL elements, the luminescence production operation is performed according to the flow of current to the organic EL element included in each pixel, and thus the power consumption of the display panel tends to increase compared to a liquid crystal element which is a voltage-driven element. In particular, the power consumption of the display panel increases following increases in screen size and level of high-definition.
Japanese Unexamined Patent Application Publication No. 2008-89726 (Patent Reference 1) discloses a circuit configuration that reduces the power consumption of pixel units in an active-matrix organic EL display device.
FIG. 17 is a circuit diagram showing an example of a specific circuit configuration of a pixel circuit included in an organic EL display device disclosed in Patent Reference 1. As shown in the figure, a luminescence pixel 100A includes: a selection transistor 121b for writing the voltage of a data line 112 into a holding capacitor element 124b when the luminescence pixel 100A is selected according to a scanning signal of a scanning line 111; the holding capacitor element 124b; a P-type drive transistor 122 which supplies a drive current corresponding to the held voltage of the holding capacitor element 124b, from a high-luminance power source line 113 or a low-luminance power source line 114; and an organic EL element 125 which produces luminescence according to the flow of such drive current. The above-described pixel configuration is a configuration that is included in a normal pixel circuit.
In addition, the luminescence pixel 100A includes: a switching transistor 123 turns the high-luminance power source voltage from the high-luminance power source line 113 ON and OFF; a diode 126 which turns the low-luminance power source voltage from the low-luminance power source line 114 ON and OFF; a holding capacitor element 124a which has one terminal connected to the high-luminance power source line 113 and the terminal connected to the gate of the switching transistor 123; and a selection transistor 121a which has a gate connected to the scanning line 111, and inputs a control signal VELS to the gate of the switching transistor 123 when the luminescence pixel 100A is selected according to the scanning signal from the scanning line 111. The source of the switching transistor 123 and the cathode of the diode 126 are connected, and the source of the P-type drive transistor 122 is connected to such common connection point.
The above-described switching transistor 123, selection transistor 121a, holding capacitor element 124a, and diode 126 compose a power source voltage switching unit for switching between the use of either the high-luminance power source voltage or the low-luminance power source voltage, as the pixel power source voltage to be supplied to the P-type drive transistor 122.
In the above-described circuit configuration, when the high-luminance power source voltage is selected, the scanning signal and the control signal VELS simultaneously switch to the high level in the writing period. In this case, the switching transistor 123 turns ON, and the high-luminance power source voltage is supplied to the source of the P-type drive transistor 122. At this time, the diode 126 becomes reverse-biased and automatically turns OFF because the anode potential becomes the low-luminance power source voltage level and the cathode potential becomes the high-luminance power source voltage level, and thus the power source voltage from the low-luminance power source line 114 is cut-off.
On the other hand, when the low-luminance power source voltage is selected, the scanning signal switches to the high level and the control signal VELS stays in the low level in the writing period. In this case, the switching transistor 123 turns OFF, and the power source voltage from the high-luminance power source line 113 is cut-off. At this time, the diode 126 becomes forward-biased and turns ON, and the low-luminance power source voltage is supplied to the source of the P-type drive transistor 122.
As described above, in the circuit configuration illustrated in FIG. 17, the diode 126 is turned ON and OFF by turning the switching transistor 123 ON and OFF according to the control signal VELS.
Here, with regard to the control signal VELS, the scanning line drive circuit to which the scanning line 111 is connected determines the voltage level in the manner described below. For example, in the case where entire display grayscale is expressed by 256 levels, the control signal VELS is switched to the high level to select the high-luminance power source voltage when the grayscale signal value of the luminescence pixel 100A belongs to the high grayscale-side when the grayscale level 128 is assumed as a standard value, and the control signal VELS is switched to the low level to select the low-luminance power source voltage when the grayscale signal value belongs to the low grayscale-side.
According to the above-described configuration, the organic EL display device disclosed in Patent Reference 1 is provided with a high-luminance power source voltage and a low-luminance power source voltage, and controls switching of pixel voltage individually for each pixel circuit according to the control signal VELS, and, accordingly, has a circuit configuration that reliably prevents deterioration of picture quality and at the same time reduces power consumption.